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Vitamin C, also known as L-ascorbic acid, is found in citrus and soft fruit and in leafy green vegetables such as broccoli, peppers, Brussels sprouts and sweet potatoes. Vitamin C is essential for the synthesis of collagen, which is a structural protein of skin, connective tissue, tendon cartilage and bone. Without vitamin C in the diet, humans would get the disease, scurvy, which results in weak blood vessels hemorrhaging, loosening of the teeth, lack of ability to heal wounds and finally death. Humans, monkeys, guinea pigs and a few other vertebrates lack the enzymes that are essential for the biosynthesis of ascorbic acid from glucose, hence it needs to be included in the diet.

Solubility of Vitamins

Vitamins are either water soluble or fat soluble, depending on their molecular structures. Water-soluble vitamins have many polar groups, so they are soluble in polar solvents such as water. Fat-soluble vitamins are predominantly non-polar and are soluble in non-polar solvents such as the fatty tissue of the body.

Molecular Structure of Vitamin C

The molecular structure of vitamin C resembles that of the five-ringed monosaccharide, ribose, although vitamin C has several additional features. Firstly, the five-membered carbon ring of vitamin C is unsaturated, meaning that two hydroxyl (OH) groups are attached to double-bonded carbon atoms. This is not the case with the ribose structure, in which each carbon atom (C) is saturated with hydrogen atoms (H), so that two single bonds exist instead of one double bond. Furthermore, carbon one of the vitamin C molecule is unsaturated, with the carbon atom double bonded to the oxygen atom. Again, in the ribose molecule, that double bond does not exist due to saturation of the carbon atom with hydrogen atoms.

Physical Properties of Carbohydrates

Nonetheless, vitamin C is classified as a carbohydrate. The chemistry of carbohydrates is mainly the combined chemistry of two functional groups: the hydroxyl (OH) group and the carbonyl (-CHO) group, both of which are water soluble. Solubility of these groups in water arises because both water and these functional groups are polar molecules, meaning that they posses a positive and negative charge. Because opposites attract, when we introduce two polar substances together, they will become attracted to one another, with the positive pole of one molecule binding to the negative pole of the other molecule. This is dissolution.

In the case of the hydroxyl (OH) functional group, the oxygen atom is more electronegative than the hydrogen atom, so it has a strong tendency to pull electrons in a hydrogen-oxygen bond toward itself. This makes the oxygen atom negatively charged and the hydrogen atom positively charged. This is also the case with the oxygen and hydrogen atoms of the water molecule. When mixed together, a negatively charged oxygen atom in water will attract a positively charged hydrogen atom of the hydroxyl group, separating it from its own oxygen atom and dragging it into the aqueous phase.

In the case of the carbonyl (-CHO) functional group, oxygen is again more electronegative than carbon so pulls electrons in a carbon-oxygen bond towards itself. Additionally one of the two pairs of electrons that make up a carbon-oxygen double bond is even more easily pulled towards the oxygen, thus making the carbon-oxygen double bond very highly polar.

Physical Properties of Vitamin C That Differ From Those of Carbohydrates

Vitamin C actually lacks the carbonyl (-CHO) functional group, but it is no less soluble in water, as the hydrogen of the hydroxyl group at carbon three is acidic, thereby making it more than 1 billion times more likely to ionize than a simple OH group. What is meant by acidic is that once the hydrogen has left the molecule (ionized), the remaining negatively charged oxygen molecule will spread its negative charge evenly between that oxygen at carbon three and the oxygen at carbon one, creating a resonance structure known as a resonance stabilized ascorbic anion. Resonance structures are more stable than straightforward ions, making such molecules much more likely to ionize, thereby increasing their solubility in water.

About the Author

Annabelle Charbit has been writing since 2006, with her first play performed by CP Theatre Productions in London. She has also been published in "The London Paper" and the British Neuroscience Association’s Bulletin. Charbit holds a Doctor of Philosophy in neuroscience from University College London.